accounts: add API-controlable "active" property

* add "active" runtime property to accounts
* accounts can only be used when active
* "active" state is not persistans
* add "active" state to volatile properties map
* add setAccountActive API
* "active" is the default state (backward compatibility)

The active state can be used to manage runtime states
defining if an account should be used (e.g. on Wifi/3G etc.)

Tuleap: #9
Tuleap: #42
Change-Id: Ie3b3aaadea6e43237fc22f6a1f96de8cbfe2bd95